Hot gas path components in gas turbines typically employ air convection and air film techniques for cooling surfaces exposed to high temperatures. High pressure air is conventionally bled from the compressor and the energy of compressing the air is lost after the air is used for cooling. In current heavy duty gas turbines for electric power generation applications, the stationary hot gas path turbine components, i.e., the nozzles and turbine bucket shrouds, are attached directly to massive turbine shell structures and the shrouds are susceptible to bucket tip clearance rubs as the turbine shell thermally distorts. That is, the thermal growth of the turbine shell during steady-state and transient operations is not actively controlled and bucket tip clearance is therefore subject to the thermal characteristics of the turbine. Bucket tip clearance in these heavy duty industrial gas turbines is typically determined by the maximum closure between the shrouds and the bucket tips (which usually occurs during a transient) and all tolerances and unknowns associated with steady-state operation of the rotor and stator.
Steam cooling of hot gas path components has been proposed, utilizing available steam from, for example, the heat recovery steam generator and/or steam turbine components of a combined cycle power plant. Where steam is utilized as the coolant for gas turbine components, there is typically a net efficiency gain inasmuch as the gains realized by not extracting compressor bleed air for cooling purposes (typically in an open-cycle configuration) more than offset the losses associated with the use of steam as a coolant instead of providing energy to drive the steam turbine. The steam cooling concept is even more advantageous when the steam coolant is provided in a closed loop whereby the heat energy imparted to the steam as it cools the gas turbine components is recovered as useful work in driving the steam turbine.
Because of the differences in heat transfer characteristics between air and steam, it would be expected that turbine components designed to utilize these two cooling mediums would be constructed differently. For example, a turbine nozzle designed to be cooled by closed-loop steam cooling would be expected to be substantially different from a nozzle cooled by open-cycle air cooling. The internal passages which provide convection cooling would be shaped differently and, whereas in the case of steam cooling the coolant would be recovered from the nozzle to provide useful work elsewhere, in the case of air cooling, the air would typically be discharged through holes in the walls of the nozzle partitions to form a coolant film over the cooled component.
For a gas turbine to have the flexibility to be cooled with either air or steam (a feature of the present invention described below), it is necessary to provide the ability to interchange certain components (those to be cooled) to accommodate the different cooling mediums. A customer purchasing a simple cycle gas turbine power plant, for example, would almost certainly need to have the turbine components cooled by air, there being no available source of alternative coolant. If, however, the customer later expands his plant to an uprated combined cycle plant, steam becomes readily available as a coolant and it would to be advantageous, from an efficiency point of view, to utilize this steam to cool the turbine, necessitating a change in at least some of the hot gas path components. Removal of the stationary hot gas path components for maintenance and replacement in respective air and steam-cooled turbines typically involves major downtime and costs. Additionally, in the case to steam cooling, direct connection of steam cooling pipes between the actively cooled hot gas path components and a single turbine shell make component removal impossible without rotor removal or an overly large shell diameter. Further, cost-effective maintenance and repair of gas turbines requires change-out of all hot gas path components without rotor removal.
Thermodynamic performance of a gas turbine is a primary characteristic in determining the economic value of the turbine. Turbine bucket tip clearance is a primary contributor to improved thermodynamic performance. In current practice, the stator components are mounted on a single turbine shell. Turbine shell distortion caused by thermal and mechanical loads manifests itself as circumferential variation in radial location of the bucket shrouds and nozzle diaphragms. This circumferential asymmetry is currently accounted for by increased bucket tip to shroud operating clearances as noted previously. This has a very substantial negative impact on thermodynamic performance. Consequently, there is a need to minimize the variation in radial clearance between the shroud and bucket tips to improve turbine performance. Tip clearance control becomes even more critical with steam cooling due to the possibility of steam leakage into the hot gas path due to rubs.
It will be appreciated that a bucket utilizing a closed circuit cooling system returns all of the thermal cooling medium to be used elsewhere in the system without dispersing it into the hot gas path as in an air-cooled system. This increases the difficulty of cooling the bucket tip. Therefore, the bucket tip cap must be significantly thinner than in an open circuit cooling design to enhance conductive cooling of the tip. The reduced tip thickness increases the likelihood that a rub against, or contact with, the shroud would penetrate the cooling passages, causing evacuation of the cooling medium and potential bucket failure. Consequently, tip clearance control, particularly in a closed circuit cooling design where the components are readily removable, is of critical importance.